Dental plaque (biofilm) has been implicated as a primary causative agent of adult periodontal disease. The key event leading to initiation of the disease is the transition from commensal dental biofilm to pathogenic biofilm. It is well known that the process of the transition is involved in the colonization of several specific periodontal pathogens such as Porphyromonas gingivalis. Our long-range goal is to understand events and factors leading to the transformation of healthy plaque to pathogenic plaque and to change the course of development of periodonpathogenic biofilm by preventing attachment of P. gingivalis. In our ongoing studies, we have identified several environmental factors that can influence expression of fimA gene, a virulence gene encoding a major protein unit (fimbrillin) of fimbriea. One of the striking findings is that the presence of Streptococcus cristatus molecule(s) could significantly repress fimA expression in P. gingivalis at the transcriptional level. As a result, S. cristatus could inhibit the formation of P. gingivalis biofilm in vitro. In this grant proposal, we will put our focus on characterization of S. ctristatus signaling molecule, biochemically and genetically. The hypothesis for this proposal is that S. cristatus plays an important role in impeding P. gingivalis' colonization on dental biofilm through intergenric signaling systems. To test this hypothesis, we will start with identification and purification the signaling molecule(s) of S. cristatus. The signaling molecule will be characterized in the terms of functional and genetic structures. We will also attempt to understand regulation of the signaling gene expression in oral biofilm. Therefore, the signaling gene of S. cristatus will be cloned. The promoter region of the gene will be fused with the reporter gene such as chloramphenical acetyltransferase gene, and level of the gene expression will be determined by measuring enzymatic activity. Finally the role of this molecule in the formation of pathogenic oral biofiim will be investigated. Studies will be initiated to determine the distribution of the signaling molecule in the dental plaques from healthy subjects and pedodontitis patients. Our ultimate goal is to convert the knowledge gained from these laboratory studies to practical technology that may be used to reprogram development of the dental biofilm and to reduce the incidence of adult periodontitis.